1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus in which ultrasonic waves are transmitted into the subject, the ultrasonic waves reflected within the subject are received to obtain received signals, and an image is displayed in accordance with the received signals thus obtained.
2. Description of the Related Art
Hitherto, an ultrasonic diagnostic apparatus, in which ultrasonic waves are transmitted into the subject, particularly the human body, the ultrasonic waves reflected on tissues within the subject are received to obtain received signals, and an image of the inside of the subject is produced in accordance with the received signals thus obtained, has been widely used for the purpose of diagnoses of a disease of the inside of the subject.
Such an ultrasonic diagnostic apparatus has usually a function of producing images to be altered at high frame rate, and a display unit of a non-interlace scheme capable of displaying the images altered at high frame rate. Further, this type of ultrasonic diagnostic apparatus has usually also a VTR (Video Tape Recorder) for recording the images obtained. Such a VTR records and reproduces images at a frame rate suitable for the usual television in accordance with an interlace scheme suitable for the usual television.
According to the conventional ultrasonic diagnostic apparatus as described above, it is possible to directly transmit and display an image of update obtained through transmission and reception of ultrasonic waves, or alternatively to temporarily store images in a VTR and then read the images from the VTR to display the same on a display unit.
FIG. 10 is an explanatory view used for the explanation of a method of transformation from images according to an interlace scheme, which images are recorded in the VTR, into images according to a non-interlace scheme.
As mentioned above, the VTR records images according to an interlace scheme in compliance with a scheme of the usual television. On the other hand, as display units, usually, there is provided a display unit for displaying images according to a non-interlace scheme. For this reason, when images according to the interlace scheme are read from the VTR to display on the display unit, there is a need to transform images according to the interlace scheme to images according to the non-interlace scheme. Hitherto, the transformation is performed in accordance a way which will be described in conjunction with FIG. 10.
In case of images according to the interlace scheme, odd number line images Om (m=1, 2, . . . ), which are a set of images of the odd number lines of the scanning lines of a television, and even number line images Em (m=1, 2, . . . ), which are a set of images of the even number lines of the scanning lines of the television, are alternately repeated. In this case, alternately repeated odd number line images and even number line images are paired with one another, respectively, to generate images according to the non-interlace scheme for a display. Specifically, as shown in FIG. 10, an odd number number line image O.sub.1 and an even number line image E.sub.1, which are read in synchronism with a read vertical synchronization signal shown in part (A) of FIG. 10, are combined to generate a display image O.sub.1 +E.sub.1 according to the non-interlace scheme. The display image O.sub.1 +E.sub.1 according to the non-interlace scheme is displayed in synchronism with a display vertical synchronization signal shown in part (C) of FIG. 10. Then the even number line image E.sub.1 and the next odd number line image O.sub.2 are combined to generate a display image O.sub.2 +E.sub.1 according to the non-interlace scheme. The display image O.sub.2 +E.sub.1 according to the non-interlace scheme is displayed. Further, the odd number line image O.sub.2 and the next even number line image E.sub.2 are combined to generate a display image O.sub.2 +E.sub.2 according to the non-interlace scheme. The display image O.sub.2 +E.sub.2 according to the non-interlace scheme is displayed. This is the similar hereinafter.
By the way, in the event that the display unit according to the non-interlace scheme and the VTR according to the interlace scheme are provided, and images according to the interlace scheme read from the VTR are transformed to images according to the non-interlace scheme in accordance with a way as shown in FIG. 11 and then displayed, the following problem arises.
FIGS. 11(A)-11(D) are explanatory views used for the explanation of problems involved in transformation from images according to the interlace scheme to images according to the non-interlace scheme.
Usually, an ultrasonic diagnostic apparatus has a mode in which images are produced at a frame rate higher than that of the usual television so that a fast movement of tissues and the like within the subject can be observed. Hereinafter, there will be described problems involved in, for example, a case where images are produced at the frame rate (60 frames /second) which is twice as many as that (30 frames/second) of the usual television.
FIG. 11(A) is a typical illustration showing adjacent two frame images which are produced at intervals of 1/60 seconds in accordance with the non-interlace scheme. In case of FIG. 11(A), on the image of the left hand of the two images, there appear vertically two lines (expressed by the continuation of circle .largecircle. marks). On the other hand, on the adjacent image of the right hand of the two images, there appear the two lines (expressed by the continuation of circle .largecircle. marks) at the respective positions slightly shifted, as a result of the fact that the vertical lines move to the side during 1/60 seconds.
In order for recording onto the VTR, frame images (each consisting of a pair of odd number line image and even number line image) are produced at intervals of 1/30 seconds. Accordingly, as shown in FIG. 11(B), only the odd number lines are extracted from the frame image of the left hand of FIG. 11(A) to produce an odd number line image (an image of the left hand of FIG. 11(B)). On the other hand, only the even number lines are extracted from the frame image of the right hand of FIG. 11(A) to produce an even number line image (an image of the right hand of FIG. 11(B)). Thus, images according to the interlace scheme, which consist of repetition of the odd number line image and the even number line image produced in the manner as mentioned above, are recorded onto a VTR.
Next, as shown in FIG. 11(C), images according to the interlace scheme, which are recorded on the VTR, are read from the VTR, and the odd number frame image and the even number frame image thus read are combined to produce an image according to the non-interlace scheme. The image thus produced is a composition of two images (FIG. 11(A)) according to the interlace scheme, which are mutually shifted in time by 1/60 seconds. Consequently, as shown in FIG. 11(D), this brings about such a result that a line, which is to be essentially a straight line extending vertically, appears as a line shaped in zigzags between the odd number lines and the even number lines. Thus, there is raised such a problem that a zigzag is conspicuous and as a result the image is hard to see. When it is intended that a quick motion of tissues within the subject is observed, transmission and reception of ultrasonic waves are narrowed down to an area of interest to reproduce the quick motion, so that a high frame rate of image can be obtained. In this case, when a high frame rate of image is obtained for the purpose of an observation of the quick motion, a rate of thinning is increased with higher frame rate for the purpose of an observation of the quick motion, since the frame rate of the VTR is fixed, for example, on 30 frames/sec. Thus, when an image once recorded onto the VTR is reproduced, the phenomenon (which is referred to as a false image) as in FIG. 11(D) will be more conspicuous.
As a case where the false image is conspicuous even if a frame rate is relatively law, there is raised a so-called scroll image. A scroll image means an image representative of changes with time as to information on a certain point within the subject, or information on a certain scanning line within the subject, wherein the image is scrolled in a time axis direction. Typically, the scroll image means, for example, a scrolled image in which reflection intensity of ultrasonic waves on the respective points of a certain scanning line within the subject is expressed on a vertical axis, while a time axis is given in the form of a horizontal axis, and the image is representative of changes with time as to reflection intensity of ultrasonic waves on the respective points of the certain scanning line. In such a scroll image, old portions of the image on a time basis are sequentially erased, the image is shifted along the horizontal axis (in the time axis direction) while new portions of the image on a time basis are sequentially added.
In such a scroll image, false images are often to be conspicuous because it changes with time.